Developing small electronic devices with high functionality is requiring a high-density printed circuit board. To respond to the request, a multi-layer printed circuit board having a plurality of printed circuit boards laminated is being developed.
Moreover, in recent years, the amount of information which an electronic device such as a notebook-type personal computer, a digital camera, a mobile communication device and a game machine handles is largely increasing with a tendency of faster and faster signal transmission rate. For example, in the case of personal computers, the transmission rate was moving to the transmission standard with 6 Gbps of transmission rate from year 2010 to year 2011. Due to this, in designing a printed circuit board, it is important more and more for signal loss on the transmission line to be taken into consideration.
To be compatible with further increasing signal rate and longer transmission line, it is discussed that liquid crystal polymer (LCP) is applied to the insulative layer of the flexible printed circuit board. Since the liquid crystal polymer has low dielectric constant and dielectric tangent (tan δ), allowing small dielectric loss, the transmission loss can be reduced.
However, the liquid crystal polymer has a larger coefficient of thermal expansion in the thickness direction as compared with an insulative material used for the conventional flexible printed circuit board (polyimide or the like). Due to this, it has a large difference in coefficient of thermal expansion from plated through holes conventionally typically used for inter-layer connection paths, which causes a concern that connection reliability with respect to a temperature cycle and the like cannot be sufficiently secured. As the liquid crystal polymer layer is made thicker in order to reduce the transmission loss, the connection reliability is more difficult to be secured.
Therefore, in place of the plated through holes, it is proposed that the inter-layer connection is made with conduction vias formed by filling bottomed via holes with conductive paste (Patent Literature 1 and Patent Literature 2).
Moreover, in view of preventing air voids from entering the conductive paste, it is described that bottomed via holes are filled with conductive paste using a vacuum printer (Patent Literature 3 and Patent Literature 4).
The vacuum printer vacuum-evacuates the entire system including a base material as a printing target (boring base material in which the bottomed via holes are formed), a printing plate, a squeegee and the like. After that, it scans the squeegee to fill the inside of the bottomed via holes with the conductive paste. In this stage, there can be a case where gaps (voids) which are not filled with the conductive paste arise in the bottomed via holes. Since the gaps are mostly in a vacuum state, the gaps are eliminated by releasing the system to the atmospheric pressure after completion of the printing.
In this way, to print the conductive paste under the vacuum environment can prevent the so-called air voids from arising in the conductive paste.